1. Field of Invention
The present invention relates to a powder material for use in the pressing and sintering of soft magnetic parts and more particularly to an iron powder, ferrophosphorus powder mixture which experiences part linear shrinkage less than 2% during sintering thereof.
2. Description of the Art
The prior art such as "Phosphorus as an Alloying Element in Ferrous P/M", by P. Lindskog, et al., Modern Developments in Powder Metallurgy, Volume 10, Ferrous and Nonferrous P/M Materials, pages 97-128, copyright 1977, from the proceedings of the 1976 International Powder Metallurgy Conference, teaches that shrinkage of iron powder parts increases significantly as the phosphorus content of the powder mixture increases beyond about 0.30%. In the production of soft magnetic materials from iron powder, the phosphorus content significantly exceeds 0.30% and typically must be within the range of about 0.40 to 1.25% phosphorus. Phosphorus contents within such range enhances the magnetic properties, increasing induction for example, of the sintered iron-phosphorus alloy. Furthermore, more complete sintering is obtained with such phosphorus additions thereby beneficially increasing the density of the sintered soft magnetic part. Understandably, part shrinkage during sintering of soft magnetic parts having a phosphorus content within the range of from 0.40 to 1.25% is a problem.
It is highly desirable, from a cost and efficiency standpoint, to prepare precision magnetic parts in molds of fixed dimensions. The pressed or green part has fixed dimensions which ideally would not change during subsequent sintering. Control of shrinkage during sintering, therefore, is desired to produce parts within specified dimensional tolerances. To achieve such results, part linear shrinkage during sintering is preferably less than 2% of the die dimension. Reducing shrinkage to such close tolerances minimizes and perhaps eliminates part machining requirements prior to usage of the parts.
Recognizing that part shrinkage during sintering is a problem, initial attempts at dealing with shrinkage involved constructing dies in such a manner as to compensate for shrinkage. It was soon discovered that part shrinkage varied with each powder lot and therefore, could not be accurately predicted. Thus, additional process steps, such as re-pressing or sizing was necessary more often than not. Compensation for shrinkage also proved to be an expensive proposition because of the costs of the dies. Efforts were then directed to controlling shrinkage rather than compensating for it, as the practical solution.
The prior art such as Eisenkolb, F, Stahl und Eisen, 79 (1959) pp 1345-1352, and Bockstiegel, G, Metallurgie III, 4 (1962) pp 67-78, which were discussed in the above cited article, taught that overall dimensional change could be brought closer to zero by the additions of copper. It is really copper growth during sintering which counteracts phosphorus shrinkage. It is also taught in the above cited article by Lindskog et al. that carbon additions stabilize the dimensional change of iron-phosphorus powder during sintering. To maximize the magnetic properties of the sintered iron-phosphorus alloys, impurities in the iron must be minimized and, in particular, copper and carbon must be as low as possible. It is well known that carbon additions add strength to phosphorus containing, sintered iron parts. While strength may be desirable for structural parts, strength is not a primary concern for magnetic parts. In particular, the magnetic properties are lowered as stress is applied to the material, therefore the stress, or load, applied to the magnetic parts of this invention must be minimized.
Another reference, "The Influence of Particle Size and Phosphorus Additions on the Soft Magnetic Properties of Sintered High Purity Atomized Iron" J. Tengzelius and Sten-Ake Kvist, Hoganas AB/Sweden, presented at the Fifth Europeon Symposium on P/M, "P/M 78 SEMP 5", Stockholm, Sweden, June 4-8, 1978, includes a disclosure regarding the effect of particle size on dimensional changes of sintered iron-phosphorus soft magnetic parts. It is interesting to note that this article only discusses the effects of the size of the iron powder, and concludes that the magnetic properties of sintered iron materials may be improved by using coarse atomized iron powder.
Tengzelius et al. U.S. Pat. No. 4,090,868 and Svensson et al. U.S. Pat. No. 4,093,449 disclose that ferrophosphorus powder, having a phosphorus content in excess of 2.8% may be mixed with iron powder and sintered without experiencing the usual brittleness problems. Tengzelius et al. teach that impact strength is enhanced by controlling the quantity of impurities, such as silicon, aluminum, magnesium and titanium, in the ferrophosphorus powder. These patents also disclose the desirability of using ferrophosphorus having a small particle size preferably less than 10 microns, which is the exact opposite of the teaching of the present invention.
Lindskag et al. U.S. Pat. No. 3,836,355 pertains to an iron-phosphorus alloy powder made by blending ferrophosphorus having a relatively low, 12 to 16%, phosphorus content, and a maximum particle size of 75 micron with a substantially phosphorus free steel powder having a maximum particle size of from 100 to 500 micron. Pressing and sintering of such powder combinations, which may include carbon and copper impurities, appears to produce a high density article with satisfactory strength and without great dimensional changes, growth or shrinkage, during sintering.
Accordingly, an improved method of pressing and sintering a mixture of iron powder and ferrophosphorus powder containing from 18 to 30% phosphorus, in the production of soft magnetic parts containing from 0.40 to 1.25% phosphorus, is desired in which part linear shrinkage during sintering is less than 2%.